Fast Retransmit • If > 2 duplicate ACKs are received, a segment was probably lost • Sender assumes enough time has elapsed for out of order segments to have arrived and assembled • Retransmit segment without waiting for a retransmission timer to expire • Enter Fast Recovery

Fast Recovery • Sender has to assume data is still flowing due to the reception of duplicate ACKs • Loss was probably a rare or temporary event • Do not go into slow start, just go into congestion avoidance mode